Technical Field
The present disclosure relates to digital image processing algorithms, and more particularly, to image compression algorithms.
Description of the Related Art
Generally, an image to be displayed on a screen, for example a TV screen, is represented by digital data organized in a matrix structure representing a grid of pixels, each pixel being defined by a luminance component or several color components, for example luminance Y and chrominance Cr and Cb components. Each component is defined by a level or an amplitude for the pixel considered. Such a pixel structure, commonly referred to as “bitmap”, thus corresponds pixel by pixel to the image (then referred to as raster image or raster) which must be displayed on a screen. Generally, the pixel structure is stored in the same format as the one used for storage in the video memory of the screen. The raster thus stored in the video memory is read pixel by pixel on a line and line by line. This is referred to as a “raster scan”.
Currently, rasters used for so-called “HDTV” have a so-called “2k1k” size, i.e., 1,920 pixels per line on 1,080 lines. The frequency of raster display is generally 60 rasters per second. To send such an image signal, supplied by a TV decoder, on a hard-wire connection linking the decoder to the TV, it is necessary to perform a compression of the image signal sent by the decoder. Indeed, transmitting such an image signal without compression requires extremely high transfer speeds that are generally expensive and create electromagnetic interferences.
This is why the image signal supplied by the decoder is compressed. Currently, a conventional compression of such a signal can be carried out by applying a two-dimensional low-pass filter to the chrominance components of the image signal. However, even though the quality of the image ultimately displayed on the screen remains acceptable, high frequency information of the image signal may be lost. Furthermore, when such a compression is applied to an image raster having a so-called “4k2k” size, i.e., 3,840 pixels per line on 2,160 lines, the video part of the image remains acceptable, with the above-mentioned disadvantages, but the graphics and colored texts are degraded (blurred colors and decrease in contrasts). Applying such a compression on an even greater raster size, for example “8k4k”, becomes even more problematic.